theoretical and experimental optimization of o-band multiwavelength mixed-cascaded phosphosilicate...

Theoretical and Experimental Optimization of O-Band Multiwavelength Mixed-Cascaded Phosphosilicate Raman Fiber LasersVolume 3, Number 4, August 2011

J. Z. WangZ. Q. LuoZ. P. CaiM. ZhouC. C. YeH. Y. Xu

DOI: 10.1109/JPHOT.2011.21592631943-0655/$26.00 ©2011 IEEE

Theoretical and Experimental Optimizationof O-Band Multiwavelength

Mixed-Cascaded PhosphosilicateRaman Fiber Lasers

J. Z. Wang, Z. Q. Luo, Z. P. Cai, M. Zhou, C. C. Ye, and H. Y. Xu

Institute of Optoelectronic Technology, Department of Electronic Engineering, Xiamen University,Xiamen 361005, China

DOI: 10.1109/JPHOT.2011.21592631943-0655/$26.00 �2011 IEEE

Manuscript received May 13, 2011; revised June 1, 2011; accepted June 4, 2011. Date of publicationJune 9, 2011; date of current version June 28, 2011. This work was supported in part by theFundamental Research Funds for the Central Universities (2010121057). Corresponding author:Z. Q. Luo (e-mail: [email protected]).

Abstract:We theoretically analyze and experimentally optimize an O-band mixed-cascadedmultiwavelength phosphosilicate Raman fiber laser (MRFL). The theoretical analyses showthat the output power is insensitive to the variation of Raman fiber length when it exceedsthe optimal length. However, the pump threshold, slope conversion efficiency, and outputpower of the mixed-cascaded MRFL strongly depend on the output coupling ratio of theoutput optical coupler. When a 1064-nm Yb3þ-doped double-clad fiber laser is used topump a section of phosphosilicate fiber (PSF) in a mixed-cascaded Raman cavity, both theRaman gains of P2O5 and SiO2 in the fiber are utilized, and multiwavelength lasing around1320 nm with a wavelength spacing of 0.8 nm has been obtained. The different couplingoutput ratios and Raman fiber lengths are used to optimize the output performances of theO-band mixed-cascaded MRFL. The maximum output power of the O-band MRFL is 1.3 Wwith the coupling output ratio of 80% and the PSF length of 1 km. The experimental resultsare in good agreement with the theoretical ones.

Index Terms: Multiwavelength fiber laser, mixed-cascaded, phosphosilicate, stimulateRaman scattering, optimization.

1. IntroductionO-band (�1310 nm) fiber lasers have attracted much attention due to their great potentialapplications in optical fiber communication systems, medicine, and astrophysics. For optical fibercommunication systems, the O-band locates at the second optical communication window with lowdispersion in the standard single-mode fibers. Especially, with the fast development of wavelength-division-multiplexing (WDM) technology, the O-band laser sources can be successfully applied inaccess network and metro system. For example, bidirectional communication access systems arecommonly established by coarse 1310/1550-nm WDM technology [1]. However, the existing rare-earth-doped fiber lasers cannot cover the O-band [2]. Although one can use semiconductor opticalamplifier (SOA) to operate at O-band [1], [3], [4], most SOA-based fiber lasers provide very lowoutput power and are sensitive to environmental changes, which imposes some constraint inpractical applications. In contrast, the stimulated Raman scattering (SRS) lasers have somefundamental advantages, such as (1) the lasing waveband can be arbitrarily chosen by adjustingthe pump wavelengths; (2) relatively broad gain bandwidth [5]; (3) the Raman saturated output

Vol. 3, No. 4, August 2011 Page 633

IEEE Photonics Journal Optimization of O-Band Fiber Lasers

power can be very high. Benefiting from these advantages of Raman amplifier, one can evenrealize an O-band multiwavelength fiber laser which could find some applications in dense WDMsystems.

Most research on multiwavelength Raman fiber lasers has focused on the operation of C- orL-band [6]–[9]. The early multiwavelength Raman fiber lasers (MRFLs) usually used an array ofsemiconductor laser diodes (LDs) as the Raman pump sources [7], [8]. It is better to use a high-power Yb3þ-doped double-clad fiber laser (YDCFL) as the Raman pump which can make a MRFLmore compact and low cost. By utilizing a 1064-nm YDCFL as the Raman pump and cascadedlong-period fiber gratings as the comb-like filter, Han et al. [6] have reported an L-band MRFL withseven-order Raman shifts of SiO2=GeO2. When a 10 W/1064-nm YDCFL is used to pump threedifferent kinds of Raman gain fibers, Kim et al. [9] have successfully presented S- and C-bandMRFLs with a polarization-maintaining fiber (PMF) Sagnac loop mirror as the comb-like filter.However, most of SiO2=GeO2 MRFLs required many Raman-cascaded orders [6], [9], [10] forobtaining the desired lasing wavelength due to the small Raman frequency shift (�440 cm�1 only)of SiO2=GeO2, leading the complexity and high cost of the MRFLs. To reduce the number ofRaman-cascaded orders, a feasible solution is to use the larger Raman shift ð�1330 cm�1Þ of P2O5

in P-doped fiber instead of the SiO2=GeO2 Raman shift. Unfortunately, due to the narrow gainbandwidth of the P2O5 SRS, it is difficult for the multiwavelength generation using Raman gain ofP2O5 only.

Recently, we proposed a new mechanism for generating multiwavelength O-band lasing bycombining the advantages of the large Raman frequency shift of P2O5 and the broadband Ramangain bandwidth of SiO2=GeO2 from the same phosphosilicate fiber (PSF). We experimentallyachieved multiwavelength lasing in the O-band using a two-mixed-cascaded Raman PSF cavitypumped by a 1064-nm YDCFL [11]. However, only 3% power conversion efficiency was obtainedfrom this structure because the O-band MRFL system was not optimized. It is very important toobtain the higher conversion efficiency and greater lasing number by optimizing the laser param-eters of the MRFL. In this paper, by applying the fast and stable Newton–Raphson algorithm [12]to solve the Raman coupled-wave equations of mixed-cascaded O-band MRFL, we numericallyanalyzed the influences of the Raman fiber length and output coupling ratio on the conversionefficiency, the threshold pump power as well as the total output power. Then, based on thetheoretical optimization, we experimentally designed the two-stage mixed-cascaded O-bandMRFL and obtained an improved slope conversion efficiency of 33% and a maximum outputpower of 1.3 W.

2. Principle and Theoretical Model of O-band Mixed-Cascaded MRFLIt is well known that there are two frequency-shift peaks in the Raman spectrum of PSF, whichoriginates from the Raman scatterings of SiO2=GeO2 and P2O5, respectively. For the PSF used inour experiment, the Raman scattering spectrum is showed in Fig. 1 [13]. One can find that theRaman scattering of P2O5 and SiO2=GeO2 have their respective features: 1) The Raman scatteringof P2O5 has a large Raman frequency shift of 1327 cm�1with a very narrow bandwidth; 2) theRaman scattering of SiO2=GeO2 has an ultrabroad bandwidth ð�300 cm�1Þ, but its Ramanfrequency shift is relatively small (�495 cm�1 only). Considering their applications in MRFLs, theSRS of P2O5 has a great potential to reduce the Raman-cascaded orders, however, it is unsuitableto generate multiwavelength oscillation due to its narrow bandwidth. On the other hand, theultrabroad Raman gain bandwidth of SiO2=GeO2 is advantageous for producing multiwavelengthoscillation, but it will need more cascaded orders due to its small Raman frequency shift. Using theSRS of both P2O5 and SiO2=GeO2 in a PSF simultaneously, mixed Raman-cascaded MRFLs havebeen realized with only few cascaded orders [11]. The mixed Raman-cascaded process shouldmeet the following design rules: 1) Because of the lack of narrow Raman gain bandwidth of P2O5,the last-order Raman shift for generating multiwavelength must utilize the SRS of SiO2=GeO2

benefiting from its ultrabroad bandwidth; 2) to reduce the Raman-cascaded orders for simplifyingsystem, the SRS of P2O5 should be used as many times as possible. Complying with the above



rules, the proposed mixed-cascaded Raman process for realizing phosphosilicate MRFL can bedescribed in principle by the following equations:

1�1¼ 1�p��2

..

.

1�nþ1

¼ 1�n��2

..

.

1�outNþ1

¼ 1�N��1 (1)

where �p is the pump wavelength; �n ðn ¼ 1; 2; . . .Þ is the nth-order Stokes wavelength; and �outNþ1ðN � nÞ is the central wavelength of the desired multiwavelength oscillation. ��1 and ��2represent the Raman frequency shifts of SiO2=GeO2 and P2O5, respectively. For example, with thepump of 1064 nm, the first- and second-order Stokes waves can use the Raman frequency shift ofP2O5 (oscillate at 1239 nm) and SiO2=GeO2 (oscillate at 1320 nm) to realize an O-band MRFL,respectively.

Fig. 2 shows the principle schematic of O-band multiwavelength mixed-cascaded phosphosilicateRaman fiber laser. A 1064-nm laser is used to pump a section of PSF for providing Raman gain.FBG1 and FBG2 are highly reflective at the first-order Stokes wave 1239 nm. The 3-dB OC1 formsa highly reflective loop mirror at the second-order Stokes wave, while the 3-dB OC2 combine asection of PMF and a PC to form a comb-like filter [9]. Then the OC3 outputs a partial second-orderStokes power to obtain multiwavelength laser at 1320 nm. An additional grating (FGB3) centered atpump wavelength for fully using the residual pump power. At this configuration, since the lengths ofthese optical devices (including OCs, FBGs, PC, PMF, and WDM) are much shorter than theRaman fiber, the whole cavity length can be simply equivalent to the length (L) of PSF.

Here, the two-stage mixed-cascaded MRFL can be governed by the following steady-stateequations as

dPF=B0

dz¼ ��0P

F=B0 � �0

�1g1 PF

1 þ PB1

� �PF=B0

Fig. 1. Raman scattering spectrum of the phosphosilicate fiber [13].



dPF=B1

dz¼ � �1P

F=B1 � g1 PF

0 þ PB0

� �PF=B1 � g2

�1�2

PF2 þ PB

2

� �PF=B1

dPF=B2

dz¼ � �2P

F=B2 � g2 PF

1 þ PB1

� �PF=B2 (2)

where the superscript, BF[ and BB[ denote the forward- and backward-propagation directions ofpump or Stokes waves, respectively. P0, Pi ði ¼ 1; 2Þ represent the pump power, i th-order Stokeswave power, and �0, �i ði ¼ 1; 2Þ are the corresponding optical frequencies. �0, �i ði ¼ 1; 2Þ are thecoefficients of the fiber loss at the pump and Stokes wavelengths. The parameters gi ði ¼ 1;2Þ arethe corresponding Raman gain coefficients of P2O5 and SiO2=GeO2. In (2), we neglect thecontribution of spontaneous Raman scattering once the Raman oscillation happens. The boundaryconditions for the mixed-cascaded MRFL are given by the reflection of the mirrors and the injectedpump power

PF0 ð0Þ ¼Pin; PB

0 ðLÞ ¼ RL0 � PF

0 ðLÞ

PF1 ð0Þ ¼R0

1 � PB1 ð0Þ; PB


1 ðLÞ

PF2 ð0Þ ¼R0

2 � PB2 ð0Þ; PB


2 ðLÞ (3)

where RL0 is the reflection of the FBG3 at pump power, R0

1 and RL1 are the reflectivity of the FBG1

and FBG2 at z ¼ 0 and z ¼ L, respectively. R02 and RL

2 represent the reflectivity of the OC1broadband mirror and the PMF Sagnac loop mirror at z ¼ 0 and z ¼ L, respectively.

To solve (2) and (3), the two-point boundary problem must be transferred into an initial-valueproblem, and the correct initial values PB

i ði ¼ 0; 1; 2Þ should be obtained firstly. Many researchershave numerically simulated the normal Raman fiber lasers using some different methods [14]–[18].We apply a fast and stable algorithm in [12] which can reduce the calculating time to a few minutes.The basic idea of the fast algorithm can be simply described in the next. At the beginning, (3) can betransformed to following equations:

PF0 ð0Þ=Pin ¼ 1; PB

0 ðLÞ=PF0 ðLÞ ¼ RL

0

PF1 ð0Þ=PB

1 ð0Þ ¼R01 ; PB


1

PF2 ð0Þ=PB

2 ð0Þ ¼R02 ; PB


2 : (4)

Fig. 2. Configuration of two-mixed-cascaded O-band MRFL.



One can find that PFi ði ¼ 0; 1; 2Þ and PB

i ði ¼ 0; 1; 2Þ are correlated by the boundary condition (4).Moreover, PF

i ðLÞ and PBi ðLÞ also actually depend on PF

i ð0Þ and PBi ð0Þ by (2). Thereby, the two-point

boundary problem can transfer to exactly obtain the set of initial values as follows:

Pð0Þ ¼ PB0 ð0Þ;PB

1 ð0Þ;PB2 ð0Þ

� �T: (5)

Simultaneously, the output function OutputðPð0ÞÞ and the target output function TargetouputðLÞ arealso defined as

Output Pð0Þð Þ ¼ PB0 ðLÞ=PF

0 ðLÞ;PB1 ðLÞ=PF

1 ðLÞ;PB2 ðLÞ=PF

2 ðLÞ� �T

TargetoutputðLÞ ¼ RL0 ;R

L1 ;R

L2

� �T: (6)

Only when the output function OutputðPð0ÞÞ is equal to the target function TargetouputðLÞ, the inputinitial values Pð0Þ are the correct ones. Otherwise, the input initial values Pð0Þ are need furthermodified by applying the Newton–Raphson method mentioned in [12].

3. Numerical Simulation and Raman Cavity OptimizationIn this section, we perform the numerical simulation for the multiwavelength mixed-cascadedRaman fiber laser and try to find the optimization parameters (e.g., the optimized coupling outputratio and PSF length). In our simulation, the parameters of PSF are directly used those of the PSFprovided by Fiber Optics Research Center of Russia. The fiber core contains 13 mol. % of phos-phor, which results in a refractive index difference of 0.011. The cutoff wavelength is 1000 nm thatguarantees the single-mode behavior at the pump and Stokes-wave wavelengths. For the 1064-,1239-, and 1320-nm wavelengths, the fiber loss coefficients are �0 ¼ 1:84, �1 ¼ 1:16, and �2 ¼1:00 dB/km, respectively. The Raman frequency shifts are ��1 ¼ 495 cm�1 and ��2 ¼ 1327 cm�1.Their Raman gain coefficients (at 1060 nm pump) are g1 ¼ 0:85 and g2 ¼ 1:20 kW�1m�1. Thereflectivity of all the FBGs are approximately 99% with a 3-dB bandwidth of about 1 nm. All theFBGs and optical couplers are assumed to have the average insertion losses of 0.15-dB, whichinclude the absorption, scattering, and splicing losses generated in the fabrication process. ThePMF Sagnac comb-like mirror can usually induce a larger loss which is assumed to be 3 dB.Considering these losses as mentioned in above, the equivalent reflectivity of second-order cavitycan be estimated as RL

2 ¼ ð1� RatioÞ2 � 0:425 and R02 ¼ 0:85, where Ratio represents the output

coupling ratio of OC3. With these parameters, we can solve the ordinary nonlinear differentialequations (2) in MATLAB using the algorithm mentioned in Section 2. As shown in Fig. 3, thethreshold pump power is numerically calculated as a function of output coupling ratio under differentlengths of PSF. The threshold increases either with increase of the output coupling ratio or withdecrease of the PSF length from 1000 m to 300 m. To minimize the threshold, one can choose a

Fig. 3. Calculated threshold versus output coupling ratio for different lengths of PSF.


Vol. 3, No. 4, August 2011 37

low output optical coupler and a long PSF. The calculated total output power of O-band (�1320 nm)is plotted against the PSF length for different output coupling ratios at a pump power of 5 W, asshown in Fig. 4. It is evident that the output power is insensitive to the variation of Raman fiberlength when the length exceeds 300 m. Nevertheless, as shown in the dashed line of Fig. 4, theoutput coupling ratio can significantly influence on the output power and the optimal length (Lopt), forexample, the Lopt ¼ 198 m, Pmax

out ¼ 0:45 W for Rout ¼ 10% and the Lopt ¼ 425 m, Pmaxout ¼ 1:95 W for

Rout ¼ 70%.From both Figs. 3 and 4, one can find that the output coupling ratio has dominant effects to the

threshold and conversion efficiency, while the PSF length is insensitive when it exceeds the optimallength. Therefore, we focus on the output coupling ratio to optimize the laser output of the mixed-cascaded MRFL. In practical design, one can usually use a longer PSF than the optimal length forreasonably ignoring the influence of Raman fiber length. In the next simulations, we adopt the1000-m length of PSF (more than the optimal length).

Fig. 5 illustrates the slope efficiency of O-band lasing power with different output coupling ratios. Itshows that the slope efficiency is in proportion to the output coupling ratio, but the thresholdincreases along with the coupling ratio which can observe from both Figs. 3 and 5. The minimumslope efficiency of 7.65% is obtained at Rout ¼ 10% and the maximum slope efficiency of 61.2% isobtained at Rout ¼ 90%. However, it is not always a good choice to select the highest output

Fig. 4. Calculated total output power at 1320 nm versus PSF length for different output coupling ratios.

Fig. 5. Slope efficiency with different output coupling.



coupling ratio due to its high threshold and spectral characteristic. In fact, the optimal coupling ratiowhich can obtain the maximum output power will change with the variation of pump power. Fig. 6shows the relationship between output coupling ratio and output power for different pump powers.The higher is the pump power, the larger output ratio is needed to obtain maximum output power. Itcan be estimated that the output power will reach maximum value at an output coupling ratio of�90% when the pump power is greater than 5 W. In addition, the spectral characteristics will alsobe affected by the output coupling ratio, which will be discussed in the next section. As a result, wemust select proper parameters to form an optimal laser configuration under the different conditionsand applications.

4. Comparison With Experimental ResultsFig. 2 also shows the experiment setup of the proposed mixed-cascaded phosphosilicate MRFL. Acommercial YDCFL (IPG-YLR-20-SM) with the output wavelength of 1064 nm and the output powerup to 20 W is used as the Raman pump. The pump light was launched into the laser cavity througha 1064/1320 nm WDM. The first- and second-stage Raman cavities have been described inSection 3, and we adopt a �5.5 m PMF ð�n ¼ 4:16� 10�4Þ. The ports (a&b) of the output coupler(OC3) were used to extract the laser output from forward- and backward-propagation waves,respectively. In our experiment, the total output power represents the sum of two ports. An extraoptical coupler (OC4) was used to simultaneously monitor the output spectrum and output power.The output laser spectrum was monitored by an optical spectrum analyzer (OSA) and the outputpower was measured by an optical power meter (PM). In Fig. 2, the PMF Sagnac loop filteroperated at 1280 � 1340 nm was used as the comb-like wavelength selective element due to itsadvantages [9]. The channel spacing of its reflective or transmission spectrum can be calculated by

�� ¼ �2

�n � LM(7)

where �n and LM denote the birefringence value and the length of PMF used in the filter,respectively. The channel spacing of the 5.5-m PMF Sagnac loop mirror can be calculated to be0.8 nm. From (7), one can find that the channel spacing can be changed by varying the length ofPMF. Moreover, it has been reported [9] that each peak wavelength in its reflective/transmissionspectrum can be continuously tuned over a channel spacing by rotating the wave plates of PC inthe Sagnac loop filter.

The output coupler (OC3) utilized in the experiment can be with different output coupling ratios(e.g., 20/80, 80/20, 30/70, 70/30, 40/60, 60/40). As increasing the pump power, the first-order

Fig. 6. Total output power verse output coupling ratio at 1320 nm with different pump power.



Stokes wave reaches its threshold and increases with pump power. Then, the first-order Ramanlaser is employed as the pump source of second-order Stokes wave. As further increasing pumppower, the second-order Stokes wave occurs. Since all the FBGs have a small transmission of�1% at each central wavelength, the intracavity lasers at 1064 and 1239 nm have weak leakage atthe output ports. Fig. 7 illustrates the whole spectrum of the Raman laser at the pump power of 3.1 Wwith a 30% output coupler. It can be clearly seen that 1239- and 1320-nm Stokes waves simulta-neously oscillate in the Raman cavity, and the intensity of 1320-nm laser is obviously much strongerthan the pump power and the first-order Stokes wave.

Using different output couplers, we record four different operation states of the MRFL with thealmost same pump power as illustrated in Fig. 8. One can find from Fig. 8 that seven, six, five, andfour lasing channels around 1320 nm are obtained with an optical coupler of output ratio 20%, 30%,40%, and 60%, respectively. The wavelength spacing is 0.8 nm. The power nonuniformity and theextinction ratio are G 5 dB and 9 25 dB, respectively. Moreover, the number of lasing channels canbe further increased by boosting pump power [11]. It should be noted that the number of lasingchannels is gradually reduced with increasing the output coupling ratio. It is understood that theincrease of output coupling ratio introduces larger cavity loss, and reduces the lasing channelnumber. At the same time, using an OSA with a spectral resolution of 0.01 nm, we measure the 3-dBlinewidth of each laser channel under different pump levels. With a pump power of 1.6, 2.0, 2.46,and 2.8 W, the measured linewidth are 0.227, 0.260, 0.284, and 0.292 nm, respectively. It isinteresting to note that the 3-dB linewidth increases as increasing the pump power. This mainly isattributed to [19] 1) the narrow longitudinal mode spacing; 2) the strong four-wave-mixing (FWM);and 3) the spatial hole burning effect.

As shown in Fig. 9, the pump threshold of the MRFL were measured under different ratios ofoutput optical coupler. The solid line and squares represent the numerical results and experimentaldata, respectively. The numerical results agree well with experimental data on the condition of thelow output ratio, while the slight deviation happen in the case of high output ratio. It can beexplained that the total loss estimated in the cavity is not accurate enough and the errors would beamplified with a larger output ratio.

Under different output coupling ratios, the measured total output power at �1320 nm is plottedagainst the input pump power, as shown in Fig. 10. A slope conversion efficiency of 33% and themaximum output power of 1.3 W have been obtained at a 80% output coupling ratio and a pumppower of 5 W, which are significantly greater than the previous work [11]. We note that themeasured slope efficiency is 10% at Rout ¼ 20% and 33% at Rout ¼ 80%, while the correspondingnumerical results are 15% at Rout ¼ 20% and 54% at Rout ¼ 80%. The measured data do not well

Fig. 7. Measured output spectrum of the MRFL, including the pump, first-, and second-order Stokeswaves.



coincide to the simulation results because the estimated total loss in our numerical model is notenough accurate. Additionally, as described in Fig. 4, the conversion efficiency is insensitive tothe variation of PSF length after exceeding the optimal length. To testify this characteristic,Fig. 11 illustrates the slope conversion efficiency with different Raman fiber length at the outputcoupling ratio of 70%. It can be seen that the slope conversion efficiency is 22.5% forL ¼ 1000 m and 28.5% for L ¼ 350 m, where it is the optimal length at the Rout ¼ 70%, but thepump threshold is significantly increasing from 1.45 W at L ¼ 1000 m to 3.29 W at L ¼ 350 m,respectively.

Fig. 9. Threshold as a function of output coupling ratio at L ¼ 1000 m (Squares: Experimental data.Solid line: Numerical simulation).

Fig. 8. Multiwavelength spectra for different output coupling ratios.



5. DiscussionsAccording to the theoretical and experimental optimization in above, the design of O-band mixed-cascaded MRFL should take into account the following considerations.

1) The theoretical analyses show that the slope conversion efficiency of an O-band mixed-cascaded MRFL is insensitive to the variation of the Raman fiber length when it exceeds theoptimal length. In contrast, the output coupling ratio has significantly influenced on both thethreshold and conversion efficiency.

2) The numerical and experimental results indicate that a high output ratio of output opticalcoupler is required to obtain high conversion efficiency. As described in Fig. 5, with boostingthe pump power, a �90% output OC should be the optimal choice to get the maximum outputpower.

3) The number of lasing channels gradually decreases with increasing the output coupling ratio,as shown in Fig. 10.

In practice, for different applications, one may require different characteristics of the MRFL.Therefore, we briefly discuss how to achieve optimal structure for different applications. When aMRFL with high output power is required, using a high output coupling ratio (e.g., 80%, 90%), is agood choice. For optical fiber communication systems [20], it needs a stable MRFL with as manylasing channels as possible. For this purpose, one should adopt a lower output coupling ratio and a

Fig. 11. Slope conversion efficiency for different lengths of PSF (1000 m and 350 m).

Fig. 10. Measured output power as a function of pump power.



longer PSF for obtaining a large number of lasing channels and a lower pump threshold. Mean-while, it is also an alternate way by boosting the pump power for more lasing channels. In a word,for different applications, one should choose a proper optimal scheme for the desired applications.

6. ConclusionWe have numerically optimized the O-band multiwavelength mixed-cascaded phosphosilicateRaman fiber laser by applying the coupled-wave equations and the Newton–Raphson method. Wealso carried out experimental studies to testify the numerical results. The experimental results are ingood agreement with the theoretical ones. We have demonstrated an O-band MRFL lasing around1320 nm with a wavelength spacing of 0.8 nm, extinction ratio 9 25 dB and the power nonuniformityless than 5 dB. The maximum output power of the O-band MRFL is as high as 1.3 W with thecoupling output ratio of 0.8 and the PSF length of 1 km. In addition, taking into account both theconversion efficiency and the number of lasing channels, we have discussed how one can obtainan optimum configuration of MRFLs in special applications.

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